Patent GB-848411-A (GB)
Title: Improvements relating to control mechanisms for automatic machine tools
Inventor: SPENCER ROLF EDMUND
IPC: G05B19/4105
Filed: 1955-10-25
Abstract: 848,411. Programmed electric control-systems. ELECTRIC & MUSICAL INDUSTRIES Ltd. Oct. 18, 1956 [Oct. 25, 1955], No. 30448/55. Class 38(6). [Also in Group XXXVI] Control mechanism for an automatic machine comprises first interpolating means for interpolating along spaced plane sections L 1 , L 2 , L 3  of a three-dimensional surface 1 in response to signals representing reference points which determine the sections, and further interpolating means for interpolating within the outputs of said first interpolating means. A machine for milling a turbine or impeller blade 1 has a toolholder and a work-table and slide which can be moved in x, y, z directions. The blade root 4, which contains the origin O, is tilted by an angle S 1  out of the xy plane and the cutter 2 is similarly tilted to facilitate machining of the root surface. The co-ordinates at several points around the three cross-sections L 1 , L 2 , L 3  are recorded in a binary-decimal code on a punched card or tape. Alternatively, magnetic cards are used. The first interpolating means interpolates between points x 11 , Y 11 , x 12 , Y 12 , X 13 , Y 13  along the section and other interpolating means interpolate for general points P between corresponding points P 1 , P 2 , P 3 , the latter interpolation being effected in dependence upon a common non-geometric parameter T as des cribed in Specification 830,462. The command signals x 0 , y 0  for displacing the work in the x and y directions produced by the control mechanism and which may be recorded before using the mechanism for controlling a machine, take account of the size of the cutter 2 which has a main radius R and a tip radius r, as shown in Fig. 3, so that the locus defined by the signals is that described by the cutter centre c. The cutter size is also taken into account in a signal z 0  responsive to the displacement in the z direction, which signal is used in a sequence controller. At any instant the position of the cutter centre c with reference to the origin O is given by a vector C, where The vector n is a unit vector normal to the surface to be machined at the point P and R is a unit vector directed to the centre c of the cutter from the intersection of the vector n with the locus b. This vector equation is solved by the control mechanism which controls the cutter so that it reciprocates along the blade length and advances by small increments round the sections at the end of each lengthwise traverse, as indicated by the track KLMNOP in Fig. 4. The limits of travel are defined by a plane 7 spaced a distance r from the blade root and by a plane 8 intersecting the z axis at Z L  and lying at an angle S 2 . It can be shown that the equations which must be solved are:- (where # 1 , # 1  are two polar angles which express the direction cosines (1 n , m n , n n ) of the normal n at P, which equations are resolved by three magslip resolvers and two servo systems) and:- (m n  cos S 1 +n n  sin S 1 ) cos # 2 - In sin # 2 =0. . . 8 (which equation is solved for # 2  with two attenuators and one resolver since 1 n , m n , n n  come from the solution of equation 3). The two attenuators and the angle - S 1  can be set manually. In the illustration of the control mechanism in Figs. 5a, 5b, mechanical connections are indicated in dotted lines. Six units 11 to 16 accept input signals from a reader, the signals being derived serially or in groups from the record, and derive, in dependence upon the parameter T, the quantities indicated. Considering the typical unit 11, Fig. 6, signals representing the co-ordinates of the reference points are derived in a digital code form, and each signal representing an x coordinate of a reference point in the section L, is applied in parallel to five stores 17 to 21. The stores are controlled by a programme unit 22 which applies conditioning signals to the stores in a cyclic order, the stores being unresponsive to an applied signal unless conditioned by a conditioning signal. Thus, if the store 17 is caused to respond to the co-ordinate x 11 , the stores 18 to 21 are caused to respond to the co-ordinates x 12  to x 15 . The unit 22 and the stores 17 to 21 may be as described in Specification 807,643. Each store may be considered as an A.C. potentiometer, the tap of which is adjusted by the signal derived from the reader so that the voltage amplitude at the tap is the required analogue. These analogues are applied in order to the studs of two uni-selectors 23 and 24, each having three banks of contacts. Stepping mechanisms 23h, 24h advance shafts 23g, 24g carrying the wiper arms, whose electrical output signals are applied to quadratic interpolators 33, 37. Each interpolator is similar to that described in Specification 796,994, so that the amplitudes of the E.M.F.'s induced at the respective studs in the circle 39 represent the x co-ordinates of closely-spaced points on a quadratic curve determined by the input signals. The co-ordinate values represented by the input signals are determined in dependence upon a selected value of the parameter T, whose instantaneous value is represented by the angular displacement of a shaft 41 common to the units 11 to 16. The brush 42 picks up a stepped output signal which is an A.C. voltage having an amplitude corresponding to the x co-ordinate (x 1 ) of P 1 . The stepping of the uni-selectors 23, 24 is controlled by a switch 43 on the shaft 41, which also carries a brush 52 which picks up a voltage having an amplitude d x 1 representing - derived from two interpolating dT differentiating devices 47, 48 incorporating a tapped inductance network transformer coupled to the interpolators 33, 37. The output signals from the units 11 to 16 are applied in selected groups as indicated in Fig. 5a to four interpolators 60, 61, 62, 63, and brushes on a common shaft 66, whose angular displacement represents the value of z, pick up the values indicated. The units 11-16 and 60-63 may include means for producing linear sub-interpolation as in Specification 796,994 and the displacement of relatively high-speed shafts may represent the variables T and z and the low-speed shafts 41, 66 may be driven continuously or step-by-step by the high-speed shafts. The mechanism thus-far described produces the co-ordinates of the general point P, i.e. the components of the vector P. It is also required to generate the vector additions to obtain the locus of the cutter dx<SP>1</SP> dy<SP>1</SP> centre c. The signals - and - from the dT dT units 62, 63 are applied to the windings of a mag-slip resolver 70. The angular position of the rotor is controlled by an electro-mechanical servo 71 included in a feed-back loop. The servo-loop tends to rotate the rotor to minimize amplitude variations of the signal picked up by the rotor, and the rotor shaft 72 therefore assumes the angular displacement # 1 , thus solving the first of the equations (3) above. The shaft 72 carries the rotor winding of another dx dy resolver 73 and the signals - and -are applied dZ dZ to the stator of that resolver. The voltage induced across the rotor windings represents dx dy - cos # 1  + - sin # 1  and this is applied across dZ dZ one stator winding of a third resolver 74. The other stator winding thereof is energized by a voltage of reference amplitude representing unity. The angular displacement of the rotor of the resolver 74 is controlled by an electromechanical servo-loop 75 tending to reduce the output of the rotor winding to zero. The rotor shaft therefore assumes an angular displacement # 1 , given by the second of the equations 3 above. A voltage representing r is set manually by a potentiometer 77 wound on a toroidal core and a similar potentiometer 80 is set by the shaft 66. Sin S 1  is set in manually on a potentiometer 84, cos S 1  on a potentiometer 85, R on a potentiometer 91 and # 1  equal to S 1  on a rotor of a resolver 93. By the series of resolvers and transformers shown, and an electromechanical servo 88 which tends to solve equation (8) for # 2 , vector components are added to the x, y, z voltage signals so that leads 98, 99 and 100 accumulate signals representing x 0 , y 0 , z 0  which are analogous to the components of the vector C. The signals x 0 , y 0  are command signals and the signal z 0  is a feed-back signal used in a sequence controller to which is also fed an analogue signal representing r from the potentiometer 77. (The toroidal-core potentiometers may comprise several stages of voltage subdivision as in Specification 802,472, and the magslip resolvers may be replaced by sine and cosine potentiometers as described in Specification 740,071. Furthermore, # 2  could be obtained by a further servo-resolver or S 1  set-in manually). In the sequence controller, two motors 110, 111 (Figs. 8a and 8b) drive the T- shaft 41 and the z-shaft 66. The power for each motor is supplied by similar circuits. Thus, power for the motor 110 is from a power amplifier 112 included in an anti-hunting feedback loop comprising a tacho-generator 113 and modulators 114. The input to the amplifier is from a high-gain amplifier 115 applied across a two-way limiter 116 which determines the maximum velocity of the motor 110 and is preset by a potential divider 117. In Fig. 4, planes 7 and 8 are defined by Z 0  cos S 1  - r+y 0  sin S 1  and Z 0 =Z 1 +y 0  tan S 2  and these equations are the basis of operation of the sequence controller. This controller is considered as starting from the point K, and drives the z-shaft 66 at a preset speed, with the T-shaft stationary. During the movement, Z 0  cos S 1  is compared with r+y 0  sin S 1  until equality is established, thus denoting the arrival of the z-servo at L. A relay A is then released to cause the motor 110 to insert an increment of T by driving the shaft 41 through a double ratchet device. When the null position is reached at M, a relay B is released, which causes the motor 111 to equate Z 0